A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A buck power converter is one example of a switched-mode converter that converts the input voltage to an output voltage that is lower than the input voltage. Typically, the buck power converter is employed In applications wherein a stable, regulated voltage is desired at the output of the power converter
A buck power converter generally includes an active switch coupled to a source of input voltage. The active switch intermittently switches to provide an output voltage to a load at an output of the buck power converter. A controller regulates the output voltage by varying a duty cycle of the active switch. Depending on the duty cycle of the active switch, the output voltage may be regulated to any desired voltage between zero and the input voltage. Employing a high switching frequency in the power converter may cause ripple fluctuations in the output voltage. Conventional buck power converters, therefore, typically include a low pass output filter having an inductor and an output capacitor. The corner frequency of the output filter may be set sufficiently lower than the switching frequency of the active switch to minimize the ripple fluctuations.
Since the active switch is coupled in series with the inductor, an inductor current is established during a conduction interval of the active switch. Opening the active switch may, therefore, result in a high voltage across the active switch unless an alternative path is provided for the inductor current. The problem of inductor current may be overcome by coupling a diode between the active switch and the inductor. During the conduction interval of the active switch, the diode is reversed biased. Then, during a nonconduction interval of the active switch, the inductor current flows through the diode, transferring some of its stored energy to the load. The buck power converter, like other switched-mode power converters, thus preferably includes at least two semiconductor switches, the active switch and the diode, to execute a switching cycle.
A principal advantage of switched-mode power supplies is an improved power processing efficiency over other circuit approaches such as linear regulators. Switched-mode power supplies may also be required to operate in four quadrants (e.g., provide a bipolar output voltage or transmit power in either direction at the input or output). If four-quadrant operation is required, conventional switched-mode power supplies typically employ a bipolar source of input power and an auxiliary active switch in place of the diode. While the use of two active switches may enable four-quadrant operation, circuit complexity is increased. Further, unlike the active switches employed in the buck power converter previously discussed, the active switches of the four-quadrant power supply may be required to block or conduct currents in both directions. Since many commonly available active switches, such as field-effect transistors (FETs), provide only unidirectional current blocking, each active switch may consist of two series-coupled FETs. In addition to the increased costs of providing the two active switches, custom controllers may also be required to provide isolated drive signals to the control terminals of the active switches, further increasing the cost and complexity of the power supply.
Accordingly, what is needed in the art is a current circulation device for a power supply and, additionally, a power supply that employs the current circulation device and is thus capable of bidirectional operation, thereby overcoming the h gh cost and complexities associated with the use of the two switches of the prior art.